A well-known method of making accurate wavelengths and bandwidth measurement of laser beams is to use an etalon (also called a Fabry-Perot etalon). In a typical etalon two very flat transparent plates are held close together rigidly in place with three precision quartz or Invar spacers sandwiched between the two plates defining the spacing between the plates. The finesse of an etalon is determined by the fringe patterns it produces (on an intensity vs. frequency or wavelength plot) and is defined as the ratio of the free spectral range (the distance between the fringes) to the width of the fringes (at half maximum). A diffuse laser beam passing through the plates will produce interference patterns. In laser systems the patterns are sometimes measured using a linear fast photodiode array such as a 1024 pixel array. The array, appropriate optical equipment and a fast digital processor can be used to monitor the spectral information in each pulse of a 4,000 Hz laser beam and calculate the wavelength and bandwidth for each pulse. Accuracy of these measurements is very important since the laser beam is used as a light source for integrate circuit fabrication where feature sizes are as small as few hundred nanometers. The interference pattern can be projected onto a screen and viewed visually. Alternatively, the pattern can be focused onto a detector array such as pixels of a CCD camera.
In some cases because of errors in etalon fabrication processes or other reasons, the spacing between the plates of the etalon is not uniform. This affects the resolving capabilities of the etalon. Small errors in the spacing are difficult to measure using conventional measurement techniques.
The manufacture of an air-spaced etalon proceeds as follows: (1) the individual etalon plates are polished flat and tested using a conventional Fizeau interferometer; (2) the plates are coated with high reflective coatings for the desired wavelength and usually re-tested for flatness; (3) three matched, ultra-precision spacers are selected; (4) the two etalon plates and three spacers are optically contacted into a single assembly. While the coated plates may be relatively flat within the central clear aperture, the forces involved in the optical contacting process, plus edge rolloff in the plates, have the potential to degrade the parallelism of the assembly. In addition, there are limits to how precisely the thickness of the three spacers can be verifiably matched prior to assembly.
In a conventional interferometer, an etalon cavity is formed between a Fizeau reference flat and the test surface. An expanded, highly collimated beam from an single-mode He—Ne laser is reflected from this combination and captured by a camera. During a measurement sequence, the spacing between the reference flat and test surface is varied by about a half-wavelength by translating the reference flat with PZT actuators. Typically 5–10 steps are taken over the half-wavelength translation. The resulting interference patterns are captured by the camera, analyzed, and the phase of the interference extracted for each point on the test optic. Since the wavelength of the light is precisely known, the physical shape of the test surface can be determined (relative to the reference flat, which is usually assumed to be flat).
The finished etalon assembly cannot be tested with a conventional interferometer because the wavefront error map which is generated represents the relative spacing between the Fizeau reference flat in the interferometer and the cavity of the test etalon. What is needed is a direct measure of the spacing uniformity of the etalon cavity itself.